There is no perfect moment when you know everything, and waiting for that moment usually means never starting. A good rule is to start practicing once you understand the basic idea behind what you want to do. For example, if you are learning SEO, once you understand what keywords are and why content matters, that is enough to start writing and experimenting. Real progress happens when you try things and see what works. So in your opinion, should beginners set a time limit on learning theory before they start practicing?